1. Field of the Invention
This invention pertains to the field of ballast circuits for powering arc discharge lamps, and is more particularly directed to a fault protection circuit for turning off ballast output power to the lamp load in the event of lamp malfunction.
2. State of the Prior Art
Arc discharge lamps such as fluorescent lamps require relatively high operating voltages, particularly when first initiating the arc discharge in the lamp. Once started, the lamp current must be externally limited to a normal level because arc lamp impedance characteristically drops after start-up. Many so called ballast circuits have been devised for supplying the necessary voltages and currents for powering such lamps.
Arc discharge lamps, for example fluorescent lamp tubes, are subject to certain types of malfunction characteristic of this type of lamp. If the lamp tube is loosened from its electrical socket the lamp connectors or pins may become sufficiently exposed to sustain an arc discharge between the exposed metal parts. Such an arc discharge is undesirable not only because it can severely stress the electrical components of the ballast circuit, but may also pose a fire and electrical shock hazard. Under such circumstances, it is, of course, desirable to shut down electrical power to the lamp immediately. Another type of lamp malfunction occurs with aging of the discharge lamp, manifested by partial rectification of the A.C. current delivered by the ballast to the lamp load. In this case too it is desirable to interrupt electrical power to the lamp load in order to call the attention of maintenance personnel to the need for replacing the lamp. Ballast output protection is particularly needed in arc discharge lamp fixtures installed in aircraft cabin lighting and other vehicles where lighting fixtures are subject to shock and vibration. Lamps shaken loose from their sockets frequently result in arcing and occasionally in fires when the lamp contacts become exposed. In the interest of safety it is desirable to minimize the possibility of lamp arcing, not only on public transportation of all kinds, but wherever arc discharge lighting is used.
Various schemes have been devised for sensing abnormal operating conditions of the lamp load in arc discharge fixtures and shutting down the output of the ballast in such event. For example, it is known that certain failure modes, particularly arcing, result in abnormally high transient voltages and currents through the lamp load. This knowledge has been exploited in the past to design protection circuits responsive to such higher than normal voltages or currents. In one known type of protection circuit a voltage comparator has one input connected for sensing the lamp voltage and the other input referenced to a fixed, preset voltage threshold. The output of the comparator is connected for switching off the ballast output in the event that the lamp voltage exceeds the treshhold voltage. An improved form of this type of output protection circuit, intended for use with variable lamp loads, is disclosed in a commonly assigned copending application.
In general the sensing of lamp fault conditions in arc discharge lighting has proven to be a challenging task. Although many solutions have been proposed many are too complex or costly to implement on a large commercial scale. A successful solution must be not only dependable but also economically viable in the highly competitive and cost sensitive interior lighting industry. Furthermore, the ballast output protection must work reliably with so called universal ballasts intended to supply lamp loads of different wattage, as well as with dimmable ballasts which deliver a variable lamp current for adjustable illumination.